A mixing valve representative of the prior art is shown in my U.S. Pat. No. 2,501,657. In such valves, hot and cold water are admitted into a cylindrical mixing chamber through ports in the sidewall thereof, and the mix and flow quantity are controlled by a cylindrical plug in the chamber, the leakage between the plug and sidewall being limited either by the closeness of the fit between the plug and cylinder, or by seals. Close fits and tolerances, however, not only increase the cost of manufacture, but also increase maintenance as erosion, wear and corrosion begin to occur. Close fits also limit the choice of materials which may be used because of differences in coefficient of thermal expansion.
Assume the cylindrical valve bore and plug of my aforementioned illustrative prior art valve to be vertically oriented, and the plug to be rotatable to register a generally circumferentially extending passage or groove thereof with either one or both of two inlet ports, this passage or groove leading to the mixing chamber, and thence to an outlet port of the mixing chamber. The valve plug is also movable axially from full-open position in which this passage or groove is laterally opposite (in the horizontal plane of) the inlet ports, to a closed position in which its end seals the outlet port, the flow volume through the valve being gradually reduced to zero by this axial travel.
Assume now that the valve plug is closed, by sealing against the outlet. The circumferentially extending passage in the plug is now out of register with the inlet ports. Cold and hot water are present in the inlet ports occasionally at different pressures, and there can be a cross flow of fluid from the port at higher pressure, through the clearance space between the valve plug and the valve bore, to the port at the lower pressure. There is a further sealing problem, in that, with the valve open to flow from just one of the inlets, there can be a small leakage flow from the "closed" inlet around the valve plug to the outlet. These leakage flows can be curtailed to a degree by machining the parts with very small clearance space, but requiring expensive machine work with slight tolerances. Alternatively, seals can be used, but these are likely to be fragile and have not been problem free.
It is accordingly a general purpose of the invention to provide a mixing valve having an improved radially expansive valve plug, which exerts a radially expansive force on the walls of the valve bore, furnishing zero clearance, and maintaining this condition, notwithstanding wear, erosion, corrosion, or temperature fluctuations over a long period of time without leakage or other disorder.